Field of the Invention
The present invention relates to a developing unit, a process cartridge, and an electrophotographic apparatus.
Description of the Related Art
The operation of electrophotographic apparatuses, such as copiers, printers, and facsimile receivers, starts with uniformly charging a rotating image carrier using a charger and irradiating the image carrier with laser light to produce an electrostatic latent image. A developing unit then feeds toner to the electrostatic latent image and develops it into a toner image. The toner image is then transferred from the image carrier to a transfer substrate (recording substrate) and fixed on the transfer substrate through heating or similar, yielding an image-bearing transfer substrate. The image carrier from which the toner image has been transferred has its surface electrically neutralized and cleaned of any residual toner to get on standby for a new image formation process.
The developing unit includes a developing chamber and a toner container which contains the toner. The developing chamber is provided with some components such as a developing roller and a toner feeder that applies the toner to the surface of the developing roller. Inside the developing chamber there is also a toner regulating member, a component configured to shape the toner applied by the toner feeder to the surface of the developing roller into a thin and more uniform layer. Thus, a thin layer of the toner is sent out of the developing unit as the developing roller rotates. The thin layer of the toner adheres to the electrostatic latent image carried on the image carrier rotating in front of an exposed section of the developing roller. As a result, the electrostatic latent image is visualized into a toner image on the image carrier.
The developing unit is so designed that the toner stays in the toner container until the activation of the unit and enters the developing chamber only after the unit starts working. Before the developing unit starts to operate, therefore, the developing roller and the toner regulating member are in direct contact with each other.
Japanese Patent Laid-Open No. 8 (1996)-227212 mentions some problems caused by direct contact between a developing sleeve and a toner feed roller in a developing unit standing by for use, such as damage to the toner feed roller. The publication discloses that such problems can be solved by the use of a toner feed roller having cells in its outermost surface with a powder having a specific sort of chargeability at least on the surface of the toner feeder.
Furthermore, Japanese Patent Laid-Open No. 2007-33538 discloses that applying a powder having a glass transition temperature of 80° C. or more to the surface of the toner feeder also solves such problems.
An aspect of the invention is directed to providing a developing unit capable of suppressing a memory to leave on a developing roller, the memory being a cause of a banding in an electrophotographic image, even when held under high-temperature and high-humidity conditions for a long period of time before the start of use. The definitions of the terms “memory” and “banding” and the expression “a memory is left” are provided later.
Certain aspects of the invention are also directed to providing a process cartridge and an electrophotographic apparatus both capable of stable output of high-quality electrophotographic images.